The general argument that I have heard is that the plural suffix does not change the meaning in any way. Instead, it is grammatically conditioned when speaking about multiple entities.Personally I find that explanation a bit confusing, but if you accept it, then it should be easy enough to apply it to other languages.

Consider:dogdogs

There's no real meaning difference between those words except in number. There isn't a case where the plural means "cats". In derivational morphology, some things are irregular including in the meaning.

However, in general I don't see the point in distinguishing between "inflectional" and "derivational" because it's actually somewhat fuzzy. If you look at serious research on the subject, it's not a simple dichotomy, or at the very least the available evidence doesn't always provide enough information. The distinction should be used if it helps you to describe the language; but the description should not be used if you are forcing the data into arbitrary boxes because of it.